1. Technical Field
The described embodiment relates generally to the use of focused energy in electronics manufacturing. More particularly, devices and methods for using a radio-frequency (RF) alternating magnetic field to thermally bond adjoining components in an electronic assembly are described.
2. Related Art
A pressure sensitive adhesive (PSA) is an adhesive that bonds when pressure is applied to marry the adhesive with the adherent. An advantage to using PSA is that no solvent, water, or heat is required for activation since, as indicated by the name, a sufficient force is required to apply the adhesive to the surface. In some cases, though, an increased force may not increase adhesion. However, surface factors, such as smoothness, surface energy, and presence of contaminants may have a substantial influence on the ultimate bond strength and reliability. Moreover, PSA generally forms a reliable bond at room temperatures. As the temperature changes, however, the properties of the bond can change. For example, at reduced temperatures, pressure sensitive adhesives can experience reduced (or even loss) tack, whereas at high temperatures pressure sensitive adhesives can experience a reduced shear strength.
Therefore, in situations where bonded parts experience temperature variations that can adversely affect the PSA bond, a thermal bond film can be more desirable to use. Thermal bond films generally provide stronger and more reliable bond than PSAs. Also, thermal bond films may be desirable when narrow bond lines are used. However, in order to form a bond between the thermal bond film and the adherent, the thermal bond film must be exposed to sufficient heat for proper activation. The ability to deliver sufficient heat can be adversely affected by a number of extraneous factors, such as thermal properties of materials being bonded together, as well as materials in the thermal path between the heat source and the thermal bond film. The heat transfer rate from a heat source to a thermal bond film is inversely related to the thermal path resistance between the heat source and the thermal bond film. The thermal path resistance can be related to the thermal coefficients of the components within the thermal path, which when added together provide an overall resistance to the flow of heat from the heat source to the thermal bond film. Thus, thermal resistance can impose a much higher temperature at the thermal source than would otherwise be required. Furthermore, to achieve faster curing of a thermal bond film, a higher temperature may be required to achieve a desired thermal gradient. Having exceedingly hot elements in a bonding assembly can adversely affect components in the vicinity of the thermal path that are sensitive to high temperature conditions (such as plastics having a low melting point, or anodized aluminum susceptible to cracking).
Therefore what is desired is a method and apparatus for thermally activating an adhesive with a focused energy delivery.